A GTS is typically used to start a gas turbine engine in aerospace applications. The design of the GTS is determined in substantial measure by the relationship between torque and engine speed during starting cycles.
A starting cycle may be initiated from a cold-start condition (i.e. the engine isn't running), or from a condition in which a running engagement is required. In the latter condition, there is an impact torque (also known as "shock" torque) associated with sudden torque-transmitting engagement between the GTS and the engine. In the former condition, there is an impact torque associated with every starting cycle if the arrangement is such that the output shaft of the GTS is not constantly engaged with the input shaft of the engine.
In addition, abnormality in the design of a particular engine may result in impact torques which are unassociated with starting cycles.
Theoretically, a GTS could be designed to accommodate all conditions of torque. However, the impact torque could then result in damage to the engine rather than the GTS. Accordingly, the engine manufacturer typically requires that the output shaft of the GTS be designed to shear at a particular level of torque deemed consonant with protection of the engine. That requirement, while effective in protecting the engine, results in numerous GTS failures and associated repair and replacement cost. From the perspective of the user of the engine, the expectation of such failure also increases inventory cost.
U.S. Pat. No. 4,253,557 (Bunger) discloses a variable load clutch apparatus aimed at compensating for impact torque in the above-described applications. The apparatus incorporates a slip clutch in the form of discs 8-22. Discs 8,10,12, and 14 are connected to rotate with a clutch member 24, while discs 16,18,20, and 22 are connected to rotate with a driving member 2. The discs are pre-loaded to a degree which ensures slippage at a predetermined torque load during an impact torque condition.
The use of a slip clutch such as that proposed by Bunger is sound in principle when applied to a GTS. However, if applied to a GTS in the manner illustrated in the '557 patent, the problem of damage from impact torque is apparently replaced with the problem of excessive clutch wear. The excessive wear results from two factors. First, because the starter output shaft is not constantly engaged with the engine input shaft, the clutch slips every time the engine is started, and can therefore be expected to wear more quickly than in an arrangement which incorporates constant engagement between the shafts. Second, the illustrated arrangement contemplates a relatively low clutch-disk radius. The surface area over which slippage occurs, or through which torque transmission is effected via friction, is therefore relatively low. Consequently, the friction associated with slippage is relatively high and can be expected to result in relatively accelerated wear.
An objective of the present invention is to provide a GTS with a more wear-resistant slip clutch.
A further objective is to provide a GTS with a slip clutch which is operative to slip during an impact torque condition attending a running engagement, thus minimizing failure of GTS output shafts.